Chemical separations, chemical delivery and catalysis have all experienced improvements when accomplished at the mesoporous scale. However, widespread commercial use of mesoporous materials has been limited because mesoporous materials lack specific functionality on their surfaces. Mesoporous materials FIG. 1a are made of a solid substrate 100 that has been templated to form mesopores 102 which are pores having a diameter or characteristic cross section dimension from about 1.3 nm to about 50 nm.
Surface functionalized materials have also been proposed for chemical separations. Functional molecules FIG. 1b are formed on a substrate 110 wherein functional molecules 112 are attached to the substrate 110 with an attaching group 114, usually a siloxane group. The terminal end of the functional molecule may have a functional group 116 that binds to the chemical of interest for separation. The performance of surface functionalized materials is limited by (1) surface area of the substrate 110 and (2) the functional group density or coverage of the substrate surface area.
Attaching organosilanes on a ceramic oxide surface involves a complex series of reversible and irreversible chemical processes. In order for these molecules to bind to the ceramic surface, it is critical that either the surface or the organosilane be in appropriate (hydroxylated) chemical form to undergo the condensation chemistry necessary for the anchoring process. This can be accomplished in one of several ways, and water is critical to all of them.
In the usual method of attaching functional molecules on glass or silicon wafers, the substrate is cleaned and hydrated in such a way that virtually all of the silicon atoms on the surface bear a hydroxyl group (such a group is called a surface silanol). This level of coverage amounts to approximately 5.times.10.sup.18 silanols per square meter. In addition, due to the hydrophilicity of this interface and the fact that the cleaning process is usually carried out in aqueous media, there is usually a significant amount of water associated with the silanol interface. Exposure of such an interface to a solution of organosilanes (e.g. alkoxy, chloro, etc.) results in hydrolysis of the silane to afford the corresponding hydroxysilane, which is strongly hydrogen bound to the hydrated silica surface. This hydroxysilane then undergoes facile condensation with the neighboring surface silanol, resulting in covalent attachment of the organosilane to the silica surface. Any remaining chlorides or alkoxides on the organosilane can also undergo similar hydrolysis (secondary hydrolysis) and condensation to provide crosslinking between the silane molecules bound to the silica surface. This crosslinking significantly enhances the stability of the monolayer coating by linking adjacent silanes to one another, thereby providing secondary points of attachment.
If the cleaned silica surface is dried, then reaction of the organosilane can take place directly with the surface silanols. However, since the surface silanols are substantially less nucleophilic than water and there is a significant kinetic barrier for this reaction, this chemistry is very slow and inefficient relative to the hydrolysis/condensation chemistry described above. In addition, the lack of surface water precludes any secondary hydrolysis and condensation, thereby preventing any crosslinking. Thus, the monolayers obtained by this method are less stable and therefore of lesser quality.
The calcination step in the preparation of the mesoporous silica severely desiccates the silica surface, both in terms of adsorbed water molecules and in terms of surface silanols. All surface water is removed during the calcination step, and the vast majority of surface silanols undergo condensation to form siloxane bridges, leaving only a small number of isolated silanols. Based on our experiments, we estimate this number of remaining silanols to be about only 10% of the silicon atoms on the surface, or about 5.times.10.sup.17 silanols per square meter.
Mesoporous materials have been made according to methods set forth in U.S. Pat. Nos. 5,264,203, 5,098,684, 5,102,643, and 5,238,676 (Mobil Oil Corporation, Fairfax, Va.) as well as U.S. Pat. No. 5,645,891 (Battelle Memorial Institute, Richland, Wash.)
In the Mobil patents, a calcined silica surface is treated with an organosilane with no water present to induce hydrolysis of the silane. The only nucleophiles present capable of reaction with the silane are the small number of silanols left on the surface after the calcination process. This limits surface coverage to approximately 5.times.10.sup.17 organosilanes per square meter (approximately 10% of available silanols). In addition, since there is no water present for secondary hydrolysis and condensation, there can be no crosslinking to enhance the stability of the monolayer.
The Mobil patents report derivatizing 90% of the available silanols. It is critical to note, however, that their work was performed on a calcined silica surface with no added water. Therefore, there were very few silanols (approximately 5.times.10.sup.17 per square meter) and no water on the silica surface. They were successful in derivatizing 90% of these silanols, incorporating approximately 4.5.times.10.sup.17 silanes per square meter or only about 9% of the silicon atoms on the surface. Again, since there was no water on the surface, no secondary hydrolysis could take place, so there could be no stability-enhancing crosslinking of the monolayer.
Accordingly, there is a need for mesoporous materials with greatly increased number of the functional molecules to greatly increase the separative, catalytic and chemical delivery capability of mesoporous materials.
Chemical separations are relied upon in a wide range of industries. Various industrial, military, agricultural, medical and research activities have resulted in severe contamination, especially metal contamination, of the environment. Chemical separations are particularly useful for cleanup and remediation of contaminated waste sites. In the case of mercury, contamination may be from fossil fuel combustion; chlorine, caustic soda cement, and lime production; waste and sewage sludge incineration; and mining and benificiation operations. Contamination may be present in the air, water, sludge, sediment, and soil.
Mercury appears in three primary forms:
(1) metallic mercury: Hg.sup.0, PA1 (2) inorganic mercury: divalent mercury, Hg.sup.2+ ; monovalent mercury, Hg.sub.2.sup.2+ ; neutral mercury compounds, HgCl.sub.2, Hg(OH).sub.2 and PA1 (3) organic mercury: phenylmercury, C.sub.6 H.sub.5 Hg.sup.+, C.sub.6 H.sub.5 HgC.sub.6 H.sub.5 ; alkoxyalkyl mercury, CH.sub.3 O--CH.sub.2 --CH.sub.2 --Hg.sup.+ ; methylmercury, CH.sub.3 Hg.sup.+, CH.sub.3 HgCH.sub.3.
These compounds can be ranked in order of decreasing toxicity as: methylmercury, mercury vapor, inorganic salts of mercury and a number of organic forms such as phenylmercury salts (Mitra, Mercury in the Ecosystem, 1986, Trans Tech Publications). Methylmercury, the most toxic form, is formed mainly by methylation of mercury by methanogenic bacteria which are widely distributed in the sediments of ponds and in the sludge of sewage beds. In addition, methylation was used as a seed-dressing preparation in agriculture. The symptoms of mercury poisoning in humans includes: digestion disturbances, emaciation, diarrhea, speech stammering, delirium, paralysis of the arms and legs, and death by exhaustion.
The importance of mercury contamination is underscored by the fact that the U.S. Department of Energy (DOE) has identified the removal/separation/stabilization of mercury as the first and fourth priorities among 30 prioritized technology deficiencies in the cleanup of past weapons production activities (X. Feng, J. Liu, and G E Fryxell, "Self-Assembled Monolayers on Mesoporous Supports (SAMMS) for RCRA Metal Removal," Proceedings of the Efficient Separations and Processing Crosscutting Program 1997 Technical Exchange Meeting, Jan. 28-30, 1997, Gaithersburg, Md., p. 5.15-5.20, 1997). Over 50,000 m.sup.3 of mixed low-level and transuranic waste-containing mercury has been identified in the DOE complex. Mercury-bearing DOE wastes are aqueous and non-aqueous liquids, sludges, soils, absorbed liquids, partially or fully stabilized sludges, and debris. Many wastes, including DOE wastes, contain mercury in amounts of less than 260 ppm; these wastes are not required to be treated by retorting as specified by the Environmental Protection Agency (EPA) regulation for mercury. However, these wastes contain other contaminants that require treatment, and the presence of mercury complicates the design of offgas systems, stabilization of residues, and monitoring of all effluents. Thus, it would be advantageous to remove the mercury in a pretreatment process to simplify downstream operation.
The existing technologies for metal and mercury removal from diluted wastewater include activated carbon adsorption, sulfur-impregnated activated carbon, microemulsion liquid membranes, ion exchange, and colloid precipitate flotation. These technologies are not suitable for sludge treatment because of poor metal loading (e.g., metal uptake less than 20% of the mass of the adsorber material) and selectivity, (interference from other abundant ions in groundwater). Furthermore, they lack stability for metal-laden products so that they are not disposable directly as a permanent waste form. As a result, secondary treatment is required to dispose or stabilize the separated mercury or the mercury-laden products. Mercury removal from nonaqueous sludge, adsorbed liquids, or partially- or fully-stabilized sludges, and mercury-contaminated soil is difficult because (1) the nonaqueous nature of some wastes prevents the easy access of leaching agents, (2) some waste streams with large volumes make the thermal desorption process expensive, and (3) the treatment of some waste streams are technically difficult because of the nature of the wastes.
Mercury removal from offgas in vitrifiers and in mercury thermal desorption processes is usually accomplished through active carbon adsorption. However, the carbon-based adsorbents are only effective enough to remove 75 to 99.9% of the mercury with a loading capacity equivalent to 1-20% of the mass of the adsorber material. A last step, mercury amalgamation using expensive gold, usually is needed to achieve the EPA air release standard. A carbon bed usually is used later in the offgas system, where the temperature is generally lower than 250.degree. F. In the sulfur impregnated carbon process, mercury is adsorbed to the carbon which is much weaker than the covalent bond formed with SFMM. As a result, the adsorbed mercury needs secondary stabilization because the mercury-laden carbon does not have the desired long-term chemical durability due to the weak bonding between the mercury and active carbon. In addition, a large portion of the pores in the active carbon are large enough for the entry of microbes to solubilize the adsorbed mercury-sulfur compounds. The mercury loading is limited to about 0.2 g/g of the materials.
The microemulsion liquid membrane technique uses an oleic acid microemulsion liquid membrane containing sulfuric acid as the internal phase to reduce the wastewater mercury concentration from 460 ppm to 0.84 ppm. However, it involves multiple steps of extraction, stripping, demulsification, and recovery of mercury by electrolysis and uses large volumes of organic solvents. The liquid membrane swelling has a negative impact on extraction efficiency.
The slow kinetics of the metal-ion exchanger reaction requires long contacting times. This process also generates large volumes of organic secondary wastes. One ion exchange process utilizes Duolite.TM. GT-73 ion exchange organic resin to reduce the mercury level in wastewater from 2 ppm to be below 10 ppb. Oxidation of the resin results in substantially reduced resin life and an inability to reduce the mercury level to below the permitted level of less than 0.1 ppb. The mercury loading is also limited because the high binding capacity of most soils to mercury cations makes the ion-exchange process ineffective, especially when the large amounts of Ca.sup.2+ from soil saturate the cation capacity of the ion exchanger. In addition, the mercury-laden organic resin does not have the ability to resist microbe attack. Thus, mercury can be released into the environment if it is disposed of as a waste form. In addition to interference from other cations in the solution besides the mercury-containing ions, the ion exchange process is simply not effective in removing neutral mercury compounds, such as HgCl.sub.2, Hg(OH).sub.2, and organic mercury species, such as methylmercury, which is the most toxic form of mercury. This ion-exchange process is also not effective in removing mercury from nonaqueous solutions and adsorbing liquids.
The reported removal of metal from water by colloid precipitate flotation reduces mercury concentration from 160 ppb to about 1.6 ppb. This process involves the addition of HCl to adjust the wastewater to pH 1, addition of Na.sub.2 S and oleic acid solutions to the wastewater, and removal of colloids from the wastewater. In this process, the treated wastewater is potentially contaminated with the Na.sub.2 S, oleic acid, and HCl. The separated mercury needs further treatment to be stabilized as a permanent waste form.
Acidic halide solution leaching and oxidative extractions can also be used in mobilizing mercury in soils. For example KI/I.sub.2 solutions enhance dissolution of mercury by oxidization and complexation. Other oxidative extractants based on hypochlorite solutions have also been used in mobilizing mercury from solid wastes. Nevertheless, no effective treatment technology has been developed for removing the mercury contained in these wastes. Since leaching technologies rely upon a solubilization process wherein the solubilized target (e.g. mercury) reaches a dissolution/precipitation equilibrium between the solution and solid wastes, further dissolution of the contaminants from the solid wastes is prevented once equilibrium is reached. In addition, soils are usually a good target ion absorber that inhibits the transfer of the target ion from soils to solution.
No existing technologies have been developed for removing mercury from pump oil. Some preliminary laboratory studies of a zinc powder/filtration process was carried out at the DOE Pantex Plant showing a partial removal of mercury, but the work was discontinued.
The removal of mercury from nonaqueous liquids, adsorbed liquids, soils, or partially-or-fully-stabilized sludge at prototypic process rates has not been demonstrated. This is mainly because the mercury contaminants in actual wastes are much more complicated than the mercury systems addressed by many laboratory-scale tests that are usually developed based on some simple mercury salts. The actual mercury contaminants in any actual wastes almost always contain inorganic mercury (e.g., divalent cation Hg.sup.2+, monovalent Hg.sub.2.sup.2+, and neutral compounds such as HgCl.sub.2, Hg[OH].sub.2,); organic mercury, such as methylmercury (e.g., CH.sub.3 HgCH.sub.3 or CH.sub.3 Hg.sup.+) as a result of enzymatic reaction in the sludge; and metallic mercury, because of reduction. Since many laboratory technologies are developed for only one form of mercury, demonstrations using actual wastes are not be successful.
Treatment of mercury poisoning with mercaptan related compounds as medicines has been effectively demonstrated (Mitra, Mercury In The Ecosystem, 1986, Trans Tech).
Other metals that are of interest for remediation and industrial separations include but are not limited to silver, lead, uranium, plutonium, neptunium, americium, cadmium and combinations thereof. Present methods of separation include but are not limited to ion exchangers, precipitation, membrane separations, and combinations thereof. These methods usually have the disadvantages of low efficiencies, complex procedures, and high operation costs.
Inorganic anions are also of interest for separations and include but are not limited to TcO.sub.4.sup.-, CrO.sub.4.sup.-2, and AsO4.sup.-3. Present methods for anion separation include commercial anion exchanger resin such as Sybron.TM., JK-2, and Crypt-DER and by supramolecules such as a metalated cyclotriveratrylene. However, these methods usually have low selectivity, low capacity, and high operation cost.
Thus, in addition to the need for mesoporous materials with greatly increased number of functional molecules for binding mercury and other metals, there remains a need for materials and methods for separations that have high selectivity, high loadings, and that do not require secondary treatment. In the case of mercury, there remains a specific need for separations of mercury from complex mixed waste compounds and for mercury removal from pump oil.